ABSTRACT
From a natural history perspective, the significance of calcium in the biology of the integument (the primary barrier of an organism) is readily apparent to the comparative biologists. The higher orders of invertebrates that preceded the vertebrates essentially possess a tough, calcium-enriched exoskeleton. Crustaceans, constrained by an inflexible exoskeleton, resort to periodic molt so that body growth can progress unrestricted for a period until the new exoskeleton gets calcified. Mollusks add new growth rings to their rather inflexible shell, while some of the highly mobile members of the phylum (cephalopods) internalize the shell as a skeletal support (cuttlebone). Many of the land snails have evolved shell-lessness. Shells are energetically expensive to produce, and require a large source of calcium in the environment, so it might have been advantageous to do away with shells, as long as they developed compensatory measures, such as living in non-xerotic microhabitats. Echinoderms have a unique endoskeleton, made of individual ossicles that are porous, having an internal latticework or labyrinth-like spaces filled with dermal cells and fibers. Developmentally, the skeleton begins as numerous, separate spiculelike elements, each behaving as a single calcite crystal. An epidermis covers this dermis, internalizing the exoskeleton. Vertebrates that developed a bony endoskeleton, which has the ability to grow and keep pace with the organism’s growth dimensions, also evolved a flexible integument. The bones also provide a source of calcium that can be mobilized for the formation of eggshells (in oviparous forms) or the growing fetus. Although the vertebrate skin is not calcified in the literal sense, calcium continues to be critical for mammalian integumentary health and functions. In this chapter, we will examine some of the crucial functions of skin that are dependent on ionic calcium, and the skin dysfunctions that are correlated with altered calcium distribution and localization.
